The veterinarian with an active interest in dentistry and oral surgery will frequently encounter oral space-occupying lesions in small animal patients. A variety of neoplastic lesions occur, including both odontogenic and non-odontogenic tumor types. Non-neoplastic masses and swellings such as gingival hyperplasia and infectious conditions may be confused with oral tumors. Conversely, oral neoplasms may present as non-healing, ulcerated lesions instead of "typical" prominent masses. Of particular interest to the oral surgeon are the so-called epulides, localized swellings on the gingival margin which constitute a variety of pathological entities. Oral tumors frequently go unnoticed by the animal's owner until the tumor reaches a fairly advanced stage of development; it is therefore important to make an accurate diagnosis at the first time of presentation. Furthermore, the correct patient selection for a particular method of treatment depends on an accurate assessment of the nature and extent of the condition. Clinical staging based on the TNM-system enables the clinician to intelligently estimate the extent of the disease. The assessment should be complemented by the taking of biopsies to determine the histopathological nature of the lesion/s.

Clinical staging and biopsy

The concept of clinical staging is based on the TNM-system. Firstly, the tumor is carefully inspected and palpated. The size and site of the tumor, the presence of any ulceration and/or necrosis, and any abnormal mobility of the teeth are important findings and should be recorded. Fixation of the tumor to underlying tissues suggests bone infiltration; this possibility should be further investigated radiologically. Secondly, the regional lymph nodes are palpated to evaluate their size, shape, consistency and fixation to underlying tissues. Irregular enlargement and especially, lack of mobility are highly suggestive of lymph node involvement. Lastly, the patient is thoroughly examined to detect any signs of distant metastasis. Radiographs should be taken of the affected jaw. Intra-oral radiographs are particularly useful in cases of suspected oral neoplasia. Bone infiltration may be evidenced by varying degrees of bone resorption and/or new bone formation. However, the type of tumor can usually not be diagnosed radiologically. Thoracic radiographs are routinely indicated; other diagnostic imaging techniques may also be indicated. The data thus gathered enable the clinician to classify the patient in one of the four clinical stages. Clinical staging thus documents the extent of the condition.

The precise nature of an oral tumor is determined by the histopathological examination of a biopsy specimen; this is the mainstay of oncologic decision-making. The taking of a biopsy is indicated for all oral masses and for any suspicious lesion. Various techniques are available. A fine-needle aspirate is usually of limited value for oral tumors. An incisional biopsy using a disposable biopsy punch is recommended. For any particularly hard or bony tumor, a Michell trephine or Yamshidi needle is indicated. It is important to ensure that a representative specimen is obtained. Macroscopically normal tissue on the margin of the tumor should not be included in the biopsy, as this violates previously unopened tissue planes. The site of the biopsy should be chosen such that it falls within the boundaries of the tissue to be resected, once the diagnosis is made. In selected cases of very small tumors on the gingival margin, an excisional biopsy by means of gingivectomy may be indicated where the tumor can easily be excised in toto. A fine-needle aspirate or a cutting-needle biopsy should be taken of any enlarged lymph node.

The biopsy must be taken as atraumatically as possible to restrict the exfoliation of neoplastic cells. A properly taken biopsy has not been found to enhance the occurrence of metastasis. The biopsy should be adequately fixed and submitted to a pathologist with experience in oral pathology. The result of the histopathological examination should be compatible with the clinical findings; if not, the matter should be discussed with the pathologist. If any doubt remains, an additional biopsy may be indicated. The biopsy result allows the clinician to scientifically select the most appropriate method of treatment. A biopsy result also advances prognostication beyond the realm of intelligent guessing and the client can be more correctly informed.

Oncologic surgery

Surgical excision remains the most frequently indicated and most practical method of treatment. If surgical excision is impossible or not elected by the client, there remains the option of radiotherapy for radiosensitive tumors like squamous cell carcinoma. However, the necessary equipment is not readily available to most practitioners. When contemplating surgical treatment, it is important to have a clear understanding of the procedure's objective. In most cases, the ultimate surgical goal is to cure the patient; this is achieved by adequate excision, tumor-free margins and the absence of metastatic disease. If the extent of the disease makes this impossible, palliative surgery can be performed. The objective of palliative surgery is not to cure the patient, but to improve the quality of life and, if fortunate, achieve local control. A good example of this approach is the treatment of malignant melanoma: this tumor is known to spread at an early stage, but good local control can be achieved by radical resection of the primary tumor. Debulking is a third surgical objective: this entails removing most of the tumor prior to the application of other therapeutic modalities such as radiotherapy.

Surgical excisions can be classified according to the width of the surgical margins. It is important for the clinician confronted with an oral tumor to choose the appropriate type of excision. Surrounding the tumor are a pseudocapsule and a reactive zone; the former is a macroscopically visible membrane consisting of normal and neoplastic cells, while the latter consists mainly of inflammatory cells. An intracapsular excision involves removing the tumor from within its pseudocapsule or the piecemeal removal of neoplastic tissue. This is rarely indicated but may be acceptable for a very well differentiated odontoma which can be curetted out of the jaw bone. A marginal excision involves a dissection plane located in the reactive zone around the tumor and its pseudocapsule. This type of excision is indicated for well-differentiated, benign tumor types. Most of the odontogenic tumors fall into this category; the peripheral odontogenic fibroma, which accounts for a considerable number of the tumors presenting as epulides, is a good example. Non-neoplastic growths, such as focal fibrous hyperplasia, can also be excised in this manner. However, marginal excision is not indicated for malignant tumor types which are known to be infiltrating: not all the neoplastic tissue can be removed and almost invariably results in local tumor regrowth. These tumor types, require at least wide excision. This involves the en bloc removal of the tumor, pseudocapsule, reactive zone and a wide margin of normal tissue. This is achieved by performing a rostral maxillectomy or a segmental mandibulectomy; these procedures are indicated for small malignant tumors without bone infiltration (stages I and II), and with bone infiltration (stage III).

A radical resection involves excision of the tumor together with its supporting tissue compartment: the entire mandible, or a major part of the maxilla, and a very wide margin of normal tissue are removed. Although it may seem extreme, this approach is appropriate and necessary for malignant tumors with considerable infiltration. This category includes most malignant non-odontogenic tumor types such as squamous cell carcinoma and fibrosarcoma that involve a major part of the jaw. The peripheral or canine acanthomatous ameloblastoma, or so-called acanthomatous epulis, can also be successfully managed using wide excision or radical resection, depending on tumor size and localization.

Attention to detail in surgical technique is of great importance in oncology and the management of oral tumors. An oral tumor should be considered an "infective nidus" of neoplastic cells, all of which may exfoliate into the surgical field and give rise to local recurrence, or enter the circulation and cause distant metastasis. During surgery, an oral tumor should therefore be covered using sterile swabs and handled as little as possible; if necessary stay sutures may be placed in normal marginal tissue to facilitate manipulation. Other important measures include the tying-off blood vessels as early as possible in the procedure and electrocoagulating any accidentally exposed tumor surfaces. Once the tumor is excised, a second set of instruments and drapes should be used for the reconstructive stage of the procedure. The resected tissue fragment should be radiographed and submitted for histopathological examination; the latter should ascertain whether there are neoplastic cells present on the surgical margins.

References

1.  Gilson SD, Stone EA. Principles of oncologic surgery. Compendium Cont Educ Pract Vet 1990;12, 827-838.

2.  Owen LN. TNM Classification of Tumours in Domestic Animals. Geneva: World Health Organization, 1980.

3.  White RAS, Jefferies AR, Freedman LS. Clinical staging for oropharyngeal malignancies in the dog. J Small Anim Pract 1985;26, 581-594.

4.  Withrow SJ. The three rules of good oncology: biopsy, biopsy, biopsy! J Am Anim Hosp Assoc 1991;27, 311-314.